It is well-known that conventional cellulose ester porous membranes and polyvinyl alcohol porous membranes are usable as separating membranes for gas and/or liquid mixtures. It is also known that the gas-separating property of conventional porous membranes can be improved by impregnating the membranes with a gas-separating liquid, for example, water or an inorganic salt aqueous solution. Conventional liquid-impregnated porous membranes are, however, not always satisfactory since specific types of conventional liquid-impregnated porous membranes exhibit a poor gas-separating property in respect to certain types of gas mixtures.
Accordingly, for the purpose of obtaining a specific type of liquid-impregnated gas-separating membrane adequate for separating a specific type of gas mixture, it has been attempted to combine a specific type of polymeric porous membrane with the most adequate type of gas-separating liquid. In order to achieve this purpose, various types of gas-separating liquids have been investigated.
However, conventional polymeric porous membranes exhibit an unsatisfactory heat resistance and solvent resistance. Therefore, the gas-separating liquid to be combined with a specific type of porous membrane must be selected from a limited scope of liquids which do not dissolve therein the porous membrane. Also, the gas-separating operation must be carried out under limited conditions under which the porous membrane is not damaged.
The inventors of the present invention have made various attempts to produce aromatic polyimide porous membranes which exhibit an excellent resistance to heat and solvents. The inventors of the present invention found that the aromatic polyimide porous membranes could be impregnated with various types of gas-separating liquids which damage conventional polymeric porous membranes and that the resultant impregnated membranes exhibited an excellent gas-separating property. Especially, it was found by the inventors of the present invention that aromatic polyimide porous membranes impregnated with a specific type of gas-separating liquid exhibited an excellent gas-separating property in respect to hydrogen-carbon monoxide gas mixtures and nitrogen-oxygen gas mixtures.